Stringent controls are imposed by regulatory authorities on the emission of hazardous air pollutants, and these are likely to become even more stringent as pollution levels increase. Of particular concern are toxic trace metals and their compounds which exist in the form of small particles. Due to the nature of particulate formation in combustion processes, many of the trace metals, such as arsenic, cadmium, nickel and high-boiling point organic hazardous air pollutants, tend to concentrate on the fine, sub-micron sized particles present in flue gas.
There are various known types of devices which have been used to clean polluted airstreams by separating contaminant particles from the airstream. The known device types include “scrubbers”, cyclonic separators and electrostatic filters. Examples of such devices can be found in U.S. Pat. Nos. 4,352,681; 4,478,613; 4,853,010; 5,591,253 and 6,017,381. Some installations may utilise two or more of such device types.
Although the known devices are generally suitable for removing larger particles from airstreams, they are usually much less effective in filtering out smaller particles, particularly sub-micron particles. However, it is the smaller particles which are the most problematic pollutants. Such small particles can be hazardous to health as they are easily absorbed through the lungs.
The smaller micron and sub-micron particles also have a greater visual effect in polluted atmospheres. For the same total mass, smaller particles will generally be dispersed over a greater volume than larger particles, thereby increasing the volume of the polluted area. Further, the smaller particles cause “bending” or defraction of light, making the polluted air more visible.
Known pollution control devices tend to be rather large devices adapted for use in large factories and combustion plants. This limits their use to large scale installations. Although some smaller-sized electrostatic/cyclonic filters are known, the smaller-sized filters tend to be less efficient.
In electrostatic filters, dust particles in a gas stream are charged by corona discharge or other ionic treatment, and the charged particles are electrostatically repelled from an aperture through which the “clean” air is removed. It is believed that one of the factors contributing to the relative inefficiency of known electrostatic filters is the short “treatment” time of the particles in the ionising stage. For example, in the electrostatic filter illustrated in FIG. 3 of U.S. Pat. No. 4,352,681, the corona-producing portion 11 is a relatively short axial section which is traversed in a short period by the particles in the airstream. Consequently the particles, particularly the smaller particles, do not have sufficient time to be charged properly, and the electrostatic repulsion at the air take-off is less effective for such particles.
Another factor believed to contribute to the relative inefficiency of electrostatic filters, particularly for sub-micron particles, is the proximity of the take-off apertures to the ionisation zone, as in the embodiment of FIG. 3 of U.S. Pat. No. 4,478,613. It is believed that such proximity does not allow the smaller particles (having less charge) sufficient travel time to be moved far enough away from the take off apertures to avoid being extracted with the “cleaned” air.
It is an object of the present invention to provide an improved electrostatic filter.